A modern optical communication system (also referred to herein simply as “system”) utilizes optical amplifiers to amplify wavelength division multiplexed (WDM) signal channels as they are transmitted through the system. The first optical amplifiers to be commercially deployed were Erbium doped fiber amplifiers (EDFAs), which are self-contained units placed between 60-120 km length spans of a communication link. These units contain a special Erbium doped fiber (EDF) which serves as the gain medium used to transfer energy from laser diode pumps within the units to the optical signal channels as they pass through the unit. These amplifiers are typically operated in Automatic Gain Control (AGC) mode, where a control loop measures the signal gain of the amplifier using input and output signal power detectors and controls the laser pump (or pumps) within the amplifier to achieve the required gain.
In recent years, a new type of amplifier, namely a distributed Raman amplifier (DRA), was introduced into optical communication systems. A significant difference between an EDFA and a DRA is that for the latter, the transmission fiber itself serves as the gain medium, meaning that the signal channels are amplified as they travel through the transmission fiber. Thus, the DRA itself just provides pump power and control functionality, while the actual amplification process takes place in a distributed manner along the transmission fiber, as opposed to being lumped in a self-contained unit (as in an EDFA). This allows the length of fiber transmission spans to be increased beyond 120 km, and/or the optical signal to noise ratio (OSNR) of the system to be improved, thus allowing higher bandwidth communication.
DRAs and their applications to optical communication systems are known, see for example U.S. Pat. Nos. 6,519,082 and 6,631,025. FIG. 1 shows a schematic representation of a single transmission span 100 of a known optical communication system comprising a forward (or co-propagating) DRA 102, a backward (or counter-propagating) DRA 104 and a transmission fiber 110. In many cases, only the backward DRA is used. In some cases, the forward DRA or both backward and forward DRAs is/are used. Transmission span 100 may be preceded by an optical transmitter or by another amplifier, and may be followed by an optical receiver or by another amplifier, depending on the configuration of the optical communication system. In the case where the transmission span is followed by another amplifier, the DRA may optionally be packaged together with that amplifier, and may even have a common control unit and interface with that amplifier. The same applies to forward DRA 102, which may be packaged with an amplifier preceding it. For example, a DRA may be packaged together with an EDFA, to form a hybrid Raman/EDFA amplifier, as known in the art.
DRAs 102 and 104 include each a pump unit 106 with a high power pump laser, coupled to a Wavelength Division Multiplexer (WDM) 108. The WDM couples the pump power emitted from pump unit 106 to signal channels 112 which propagate along the span. For forward DRA 102, the forward pump power 114 is coupled to the input of transmission fiber 110 and propagates in the same direction as signal channels 112 (“co-propagating DRA”), while for backward DRA 104, the backward pump power 116 is coupled to the output of transmission fiber 110 and propagates in the opposite direction to signal channels 112 (“counter-propagating DRA”).
As the pump power propagates along the transmission fiber (either in forward or backward configuration), power is transferred to the optical signal channels, thus causing them to be amplified during their propagation along the fiber. The transfer of energy occurs due to the non-linear stimulated Raman scattering effect, as described for example in “Non-linear fiber optics”, by G. P. Agrawal, pp. 316-369, Academic Press, 2nd Edition, 1995, which is incorporated herein by reference in its entirety. In addition to signal channels amplification, noise is also created due to spontaneous Raman scattering, which is then amplified by the pump power to create amplified spontaneous emission (ASE) noise. A backward ASE noise 118 propagates in the opposite direction to signal channels 112, while forward ASE 120 propagates in the same direction as signal channels 112.
In order for the signal Raman amplification to be effective, the optical frequency of the pump lasers should be about 13 THz higher than that of the optical signal channels. Thus, if for example the optical signal channels occupy the communication band known as the C band (1525-1565 nm), the wavelength of the pump lasers should be in the range of 1420 nm-1480 nm. The term “communication band” is used to refer to the wavelength band containing the WDM signal channels transmitted by the system. The other commonly used communications band is the L band (about 1570-1610 nm). Other communication bands may be used in the future, and a single system may contain multiple communication bands. Multiple pumps with different wavelengths may be utilized to achieve uniform amplification over the entire communication band, or even over multiple bands.
In addition to the optical signal channels, some systems also include an optical supervisory channel (OSC) 122 which is transmitted along the system and is used to transmit system management information from one site in the system to another. The OSC is located in a wavelength band separate to the communication band. For example, if the communication band is the C-Band, then often the OSC is located in the 1500-1520 nm band. When a DRA is used, the OSC can also be amplified by the Raman pump power, though the gain of the OSC is typically different to that of the signal channels within the communication band.
Since for a DRA the amplification occurs along the entire transmission fiber, the signal Raman gain cannot be measured directly by measuring the signal input and output powers as in a regular lumped amplifier (such as an EDFA). This is due to the fact that at a given physical location along the link (for example at the beginning or end of the link where the DRA is placed) one does not have simultaneous access to both the signal input power and the signal output power. Furthermore, during operation, i.e. when the Raman pumps are operational and a signal is being transmitted, changes in the signal Raman gain cannot be directly measured, since changes in the signal power level can indicate add/drop of a signal channel and/or changes in the transmission line loss and not necessarily changes in the signal Raman gain. The lack of a direct method of measuring the signal Raman gain or changes in the signal Raman gain severely complicates the operation of the amplifier in AGC mode.
Until recently, most DRAs were operated in a manual mode, where the pump power is pre-set manually and the signal Raman gain is measured manually once during installation. The measured gain is then used to configure the rest of the system (e.g. the other amplifiers), and any change over time is compensated for by other amplifiers in the system. This manual mode of operation is sufficient for relatively small scale DRA deployment, but not for large-scale deployment where DRA is implemented in every span of an optical communications system. In the latter case, it is required to operate the DRA in AGC mode in order to simplify the installation and operation of the system.
There is therefore a need for, and it would be advantageous to have, apparatuses and methods for accurate gain measurement and control in DRAs. In particular, it would be advantageous to have apparatuses and methods for accurate measurement of the signal Raman gain and signal Raman gain tilt before and during DRA operation, thus allowing full AGC operation.